1. Field of Invention
The invention relates to a process unit and an image forming apparatus having a structure to remove foreign matter, such as paper dust, carried on a photosensitive member.
2. Description of Related Art
An electrophotographic image forming apparatus, such as a laser printer, is provided with a process unit that has a photosensitive drum, a charger, a scanner, a developing roller, and a transfer roller. The surface of the photosensitive drum is first uniformly charged by the charger along with the rotation of the photosensitive drum. Thereafter, the photosensitive drum is then irradiated with a laser beam emitted from the scanner and an electrostatic latent image is formed based on predetermined image data.
In the process unit, toner is stored in a toner hopper, supplied from the toner hopper to the developing roller, and carried in a thin layer on the developing roller. When the developing roller rotates and is in contact with the photosensitive drum, toner carried on the developing roller is selectively supplied onto the latent image formed on the photosensitive drum, and the latent image is transformed into a visible image. When a sheet is passing between the photosensitive drum and the transfer roller, the visible image carried on the surface of the photosensitive drum is transferred to the sheet.
In such an image forming apparatus, a cleaner-less developing method is known, in which toner remaining on the photosensitive drum after the toner has been transferred to the sheet is collected by the developing roller. For the cleaner-less developing method, if an abundance of toner remains on the photosensitive drum after transfer, the remaining toner may not be completely collected by the developing roller. Such toner may exert influence on the next visible image formed on the photosensitive drum, so that a ghost image may appear on the image.
Japanese Laid-Open Patent Publication No. HEI 9-127844 discloses a technique where an electrically conductive cleaning roller is provided so as to make contact with a photosensitive drum. When toner is transferred to a sheet, a bias is applied to the cleaning roller to move the toner remaining on the photosensitive drum to the cleaning roller, so that the toner remaining on the photosensitive drum is temporarily caught by the cleaning roller. When toner is not transferred to a sheet, that is, during a time equal to a time interval from the end of printing of a page to the start of printing of the next page, a bias is applied to the cleaning roller so as to move the toner caught by the cleaning roller to the photosensitive drum, so that the toner is returned to the photosensitive drum. The returned toner is collected by a developing roller.
However, with this cleaner-less developing method, paper dust moving from a sheet to the photosensitive drum during transfer is also caught by the cleaning roller. In Japanese Laid-Open Patent Publication No. HEI 9-127844, an electrically conductive brush that makes sliding contact with the cleaning roller is provided. When a bias with the same polarity as toner is applied to the brush, the brush catches paper dust only on the cleaning roller. However, it is difficult to cause the tip of the brush to make contact with the circumferential surface of the cleaning roller uniformly because the brush density is course. As a result, paper dust is not removed uniformly, and there is a limit in improving the ability to remove paper dust.